Add serde for Signature types (#4109)

* refactor: Added `from_slice()` method to ECDSA signatures

* doc: Modified ECDSA signature docstring to note Recovery ID

* feat: Implemented serde for Signature types

Note: using hexstring encoding

* feat: Automatically derive serde for MultiSignature

* refactor: Convert hex bytes using try_from instead of from_slice

Avoids panicking in critical code

Implemented from Peer Review

* clean: spaces -> tabs

* test: Added tests for Signature serialization

Added dependency on serde_json for testing purposes

substrate/primitives/sr-primitives/src/lib.rs

@@ -164,6 +164,7 @@ impl BuildStorage for (StorageOverlay, ChildrenStorageOverlay) {
 pub type ConsensusEngineId = [u8; 4];
 
 /// Signature verify that can work with any known signature types..
+#[cfg_attr(feature = "std", derive(Serialize, Deserialize))]
 #[derive(Eq, PartialEq, Clone, Encode, Decode, RuntimeDebug)]
 pub enum MultiSignature {
 	/// An Ed25519 signature.